Utility poles are widely used to support overhead power and telecommunication lines. Wooden utility poles are pressure impregnated before installation with materials such as creosote to minimise rotting but this still occurs, usually from the centre outwards.
The reasons for rotting usually are that
(a) the preservation does not penetrate to the centre of the poles; and PA1 (b) some soils contain chemical compounds that are particularly aggressive even towards treated timbers. PA1 (a) bonding to both sleeve and interlayer, at least in the mechanical sense of cohering or adhering with them, and preferably forming a full physico-chemical bond, and PA1 (b) allowing the load transfer from pole to sleeve via the interlayer and the core material when bending stresses are applied.
Conventionally a utility pole is sunk to a depth of around 1-1.5 meters so that the lowermost region often lies below the natural water table. Water tends to wick upwardly towards ground level, above which it is free to evaporate. Thus, some time after installation, the pole tends to become permanently saturated below ground level.
Rotting of the pole at least to some extent, is caused by spores of fungi in the atmosphere, which spores are particularly active at the higher temperatures above ground level but also in the moist environment below ground level.
According, rotting is most likely to occur at or just above ground level, the very region where the maximum bending moment is applied and therefore where the pole needs to be strongest.
Indeed, high bending stresses occur during extreme weather conditions and even new poles can be broken. For this reason poles which have lost more than 40% of their integrity (i.e. have a strength less than 60% of their original nominal strength) are replaced. This is not always easily accomplished as poles are often located in sites inaccessible to transport so that lengthy disruption of services can occur. Even though they may rot, wooden poles are still preferred in many parts of the world because of the availability of wood (and they are comparatively easily cleaned by a properly equipped workman). Alternatives to wooden poles, such as reinforced concrete and glass reinforced plastics, can also suffer damage at or about ground level.
The present invention is designed to provide a means and method for improving the in situ repair of utility poles.
For such a repair system to be viable it should be capable of reinforcing poles to a strength equivalent to that of new ones, should be easy to accomplish on site, should need access only to the base of the pole so that there is no disruption of services and should be resistant to corrosive and other attack so as to give the pole a long life without further maintenance.
A number of methods of pole repair have been the subject of patents including our own EP-A-No. 0178842. This describes a method of in-situ repairing a utility pole by providing a sleeve surrounding a substantial length of pole (below and above ground) and pouring a non-shrink hardenable pourable composition into the annulus between the pole and the sleeve so as to form a solid core bonded to both sleeve and pole.
Whilst this system works well for a time, expansion forces generated within the pole, e.g. by high moisture content, can cause the pole to swell and then the cast annular material to split and finally the sleeve material to break, especially at the joint.